package cc.wsyw126.java.nowcoder.offer360;

import java.text.DecimalFormat;
import java.util.Scanner;

/**
 * Created by dp on 17/3/25.
 */
public class E {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while (sc.hasNext()) {
            String s = sc.nextLine();
            int n = Integer.valueOf(s);
            double sum = 0.0;
            for (int i = 0; i < n; i++) {
                String s1 = sc.nextLine();
                String[] split = s1.split(" ");
                int temp = Integer.valueOf(split[0]) * Integer.valueOf(split[1]);
                sum += temp;
            }

            sum/=100;
            String s1 = formatDouble4(sum);
            if (s1.charAt(0) == '.') {
                s1 = '0' + s1;
            } else if (s1.charAt(0) == '-' && s1.charAt(1) == '.') {
                s1 = "-0" +s1.substring(1, s1.length());
            }
            System.out.println(s1);
        }
    }

    public static String formatDouble4(double d) {
        DecimalFormat df = new DecimalFormat("#.000");
        return df.format(d);
    }
}
